This invention relates to an ion mirror for a time-of-flight mass spectrometer.
Time-of-flight mass spectrometers operate on the principle that monoenergetic ions having different masses travel through a drift space at different velocities. This enables ions of different masses to be detected separately and thereby distinguished from one another.
A problem arises if, as is often the case, the ions do not all have the same energy. In these circumstances, the more energetic ions, which move at relatively high velocities, would arrive at a detector ahead of less energetic ions having the same mass. This spreading of flight times is undesirable and tends to limit the mass-resolving power of the spectrometer.
Spectrometers have been developed which incorporate so-called "time-focussing" arrangements, whose object is to reduce the spread of flight times which occurs with multi-energetic ions.
One category of "time-focussing" arrangement subjects the ions to a static electric field, and an example of this is the "reflectron", described by B. A. Mamyrin, V. I. Karatev, D. V. Schmikk and V. A. Zagulin in Soviet Physics JETP, 37 (1973)4S. The reflectron subjects the ions to a uniform electric field so as to cause their reflection. The more energetic ions penetrate deeper into the field region than the less energetic ions and, with a suitable choice of field parameters, it is possible to arrange that ions having different energies, but the same mass, all arrive at a detector at roughly the same time.
Other arrangements using static electric fields include the "spiratron", described by J. M. B. Bakker in "Advances in Mass Spectrometry" Vol. 5, p. 278, Applied Science Publishers Ltd., and the so-called "Poschenreider" device, described, for example, in German Patent No. 2,137,520.
Other kinds of "time-focussing" arrangement subject the ions to time-varying fields which have the effect of decelerating the faster ions and accelerating the slower ions with the aim of equalising the flight times of all ions having the same mass.
None of these known time-focussing arrangements is completely effective and, in practice, the flight times of ions which have the same mass do still exhibit an energy dependency, and this reduces the mass-resolving Power of the spectrometer.